/Engine/Source/Runtime/SlateCore/Public/Fonts/FontCache.h
Get the available sub-face data from the given font. Typically there will only be one face unless this is a TTC/OTC font. The index of the returned entry can be passed as InFaceIndex to the FFreeTypeFace constructor.
[TArray](API\Runtime\Core\Containers\TArray)< [FString](API\Runtime\Core\Containers\FString) > GetAvailableFontSubFaces ( const [FString](API\Runtime\Core\Containers\FString) & InFilename ) const